Compressor heat recovery
Just how much energy is out there being used to compress air? Turns out quite a bit.
The US DOE claims that total energy use for air compressors is about 90,000 GWh/yr, roughly 16% of all energy used for manufacturing in the entire US[1]. If we assume we can collect roughly 70% of that energy in the form of usable heat and convert it to electricity with existing technology, you could provide power for over 1 million Californians.
Compressing air is a notoriously inefficient process, wasting nearly 80-93% of the electrical input in the form of heat. The good news is that the majority of that heat, about 70% on average, is readily recoverable[2].
Where is the heat?
The simple act of compressing air generates the bulk of compressor heat. Usually the air is too hot to use straight out of the compressor.
Some designs will inject oil into the air stream to lubricate the moving parts. The oil will pick up some of the heat from the air before it is separated by a filter.
Both the hot air and hot oil then go through an aftercooler/oil cooler to remove heat. The two major types of aftercoolers, air-cooled and water-cooled, will dictate how much heat can be extracted.
Air Cooled Compressors
In air-cooled compressors, the hot air and hot oil are cooled by a fan, much like a car radiator. The ambient cooling air can pick up around 30 – 40 °F (17 – 22 °C) of temperature rise putting the final temperature at around 120 °F (49 °C).
Standard air compressors designs simply vent this hot air. The only system modifications needed are the addition of ducting and another fan to handle the duct loading and to eliminate any back pressure on the compressor cooling fan. These heat recovery systems can be modulated with a simple thermostatically controlled hinged vent[2].
While this waste heat is too cold to generate power even with an ORC, it is useful for either space heating or process heating. Hot air can supplement a building HVAC system to reduce the load on a furnace or it can preheat process material to reduce the load on a boiler or oven.
One drawback of air-cooled compressor heat recovery is that the compressor needs to be fairly close to the heat demand due to the inefficiencies of transporting air over long distances. In addition, care must be taken to keep the intake and exhaust for the compressor cooling air separate so as to avoid changing the pressure across the aftercooler.
Water-Cooled Compressors
The compressor sections of water-cooled compressors are very similar to their air-cooled counterparts. Instead of a radiator fan, the hot air and hot oil are cooled with a liquid, usually water. The cooling water will only pick up about 10 °F (6 °C) in temperature resulting in water temperatures around 95 °F (35 °C).
Without heat recovery, this water needs to be cooled so it can be recirculated back into the aftercooler. Cooling towers are a common way to accomplish this with a low water consumption rate. If there is not make-up water available, a radiator can be used to cool the cooling water.
While these are both great options, they both throw away the heat in the cooling water. While 95 °F (35 °C) is quite cool as far as heat recovery goes, it is still useful for space heat and process heating. Any kW of heating that can be provided through heat recovery is a kW that does not need to be purchased whether through electricity or gas.
The added benefit is that water is much easier to transport with minimal heat loss, meaning that the heat demand does not have to be located near the compressor.
Timing is Everything
One challenge with any heat recovery project is to ensure the overlap of heat availability and heat demand. If the compressors are running but there is not heat demand, then the benefit of heat recovery is not realized.
It is possible to install a thermal storage tank to buffer between heat availability and heat demand. This solution needs to be assessed on an individual basis as the added cost can make a project infeasible.
Conclusion
The opportunity for heat recovery from air compressors is massive. The US industry collectively wastes a significant amount of heat compressing air. Converting that waste heat to something useful will not be done in a single project; instead it will be done through hundreds of small projects. No waste heat recovery project is too small to profitable.
If you have any questions about a waste heat recovery project using air compressors, please send us an inquiry and we would be happy to chat with you.